1. Field of the Invention
The present invention relates to the field of targeted agent delivery and more specifically to the delivery of any therapeutic or diagnostic agent preferentially to cells expressing the CD44 receptor.
2. Description of the Related Art
Most anticancer drugs used in clinical settings are specific at a molecular rather than a cellular level. Moreover, only a small fraction of the dose reaches the target; the remaining amount of drug acts on other tissues or is rapidly eliminated. Doxorubicin is an example of one of the chemotherapeutic drugs used to treat patient with lung cancer. Doxorubicin's cytotoxicity is due to its ability to intercalate into DNA, interact with plasma membranes and take part in oxidation-reduction reactions. Thus, the cytoxic effect of this drug occurs both in normal as well as cancerous tissue and its therapeutic index is close to one. Cardiotoxicity is the major clinical adverse reaction limiting doxorubicin cumulative dosage. Toxicity and inefficient delivery remain a problem with the newer anticancer compounds being introduced into the clinic such as topotecan, gemcitabine and protein kinase C inhibitors.
Certain strategies used to deliver anticancer compounds utilize liposomes because of desirable qualities not shared by other delivery systems. Liposomes can encapsulate large quantities of drug molecules either within their aqueous interiors or dissolved into the hydrocarbon regions of their bilayers. Liposomes can also protect their contents from rapid filtration by the kidneys and degradation by metabolism, thus enhancing the drug's residence time in the body. Once taken up by a target cell, liposomes may also facilitate the cytoplasmic delivery of encapsulated drug molecules by fusing with the endosomal membrane.
So-called “conventional” liposomes have been use in cancer treatment for more than two decades to deliver a number of anticancer agents, sometimes resulting in an improved therapeutic index owing to reduced toxicity to normal tissues. However, their clinical utility has been severely limited by rapid clearance by phagocytic cells of the reticuloendothelial system (RES), by the lack of specific tumor targeting and by inappropriate release of the drug. At least two obstacles currently impede the widespread implementation of liposomes as drug carriers in vivo.
First, unmodified liposomes do not survive long in circulation, and are removed by macrophages of the reticuloendothelial system within a few hours of administration. Avoidance of this obstacle has been partially achieved by forming liposomes from saturated lipids and cholesterol, and including gangliosides or polyethyleneglycol (PEG)-derivatized lipids within the bilayer. These so-called “stealth”™ or “sterically stabilized” liposomes indeed possess reduced reactivity with serum proteins and are less susceptible to RES uptake, resulting in significant prolongation of circulation time. In addition, sterically stabilized liposomes have been shown to accumulate preferentially within tumors in animal models and in humans. It appears that the mechanism for tumor localization of liposomes involves enhanced liposome extravasation from tumor-associated vasculature, which occurs because of endothelial fenestrations and other structural abnormalities associated with tumor angiogenesis. Sterically stabilized liposomes containing doxorubicin have shown encouraging clinical activity; and doxorubicin loaded liposomes have been approved by the Food and Drug Administration (FDA) for the treatment of acquired immunodeficiency syndrome (AIDS)-associated Kaposi's sarcoma.
The second limitation is the difficulty in targeting the liposomes to tumors. Tumor-specific ligands have been difficult to identify, but even where monoclonal antibodies to specific tumor cells have been generated, rapid elimination and an immune response to the antibody have rendered the antibody-mediated targeting largely ineffectual. Further, in many instances where an antibody or other targeting ligand has been combined with the PEG coating, both long circulation and target recognition are lost. Thus, although PEG coating-appears to be necessary for prolonged liposomes survival in vivo, it simultaneously interferes with recognition of the target cell surface by the liposomes-linked antibody, forcing prior art systems to choose between targeting and survivability. Current efforts that show promise to overcome this limitation have grafted the targeting ligand unto the end of the PEG. Limitations to this approach are that a unique targeting molecule must be synthesized for each target and if the ligand is very hydrophilic it can be difficult to find a solvent that is favorable for both the ligand and the lipid.
Thus, what has been needed is a drug delivery system capable of preferentially targeting tumor cells. There is also a need for delivery vehicles that have sufficient survival in vivo to effectively deliver the payload to the desired cells. There is a further need for a drug delivery system having generic applicability to a wide range of cancers. An additional need is a drug delivery system that minimizes the cytoxicity of the drug in normal tissue while retaining its effectiveness against neoplastic tissue. This invention satisfies these and other needs.